Oxygen sensors for generating an EMF in accordance with differences in oxygen partial pressures between a sample gas such as an automotive exhaust gas and a reference gas such as air are well known and U.S. Pat. No. 3,768,259 Carnahan, et al illustrates such a device. In such devices, two media with different partial oxygen pressures P.sub.02 ' and P.sub.02 " are separated by a solid electrolyte (or ionic conductor) which has a high oxygen ion mobility. Upon attachment of conducting electrodes to both sides of the solid electrolyte, an EMF is generated which is related to the oxygen partial pressure in the two media by the Nernst equation ##EQU1## assuming oxygen can move reversibly between the media and the electrolyte. In this equation, t.sub.i is the ionic transference number, R the gas constant, T the absolute temperature, and F is the Faraday constant. Typically, the solid electrolyte used in zirconia (ZrO.sub.2) stabilized in a cubic crystal structure by calcia (CaO), magnesia (MgO), or yttria (Y.sub.2 O.sub.3) in order to obtain the maximum ionic conductivity and the lowest operating temperature. Both the magnitude and temperature dependence of the solid electrolyte's ionic conductivity, .sigma., depend critically on the concentration of the stabilizing material. Thus, in the equation for the temperature dependence of the ionic conductivity EQU .sigma. = .sigma..sub.o e.sup.-.sup.E.sbsp.a/kT
both .sigma..sub.o and the activation energy E.sub.a will depend on the concentration of the stabilizing material. For a stabilized zirconia, the best conductivity below 1000.degree. C. has been obtained with yttria at a concentration between 7 and 9 mole percent.
Existing high temperature solid electrolyte oxygen sensing devices involve the use of relatively thick (greater than 1 mm) sintered solid electrolyte materials in various configurations. Somewhat thinner sintered materials can be obtained, but only at prohibitive expense. Because the device resistance becomes very high and the time of response to a change in the oxygen partial pressure, P.sub.02', becomes very long at lower temperature, for most applications, the lower temperature limit of useful operation for a device using a sintered solid electrolyte is in the vicinity of 400.degree. C.